1. Field of the Invention
This invention relates to an electrophotographic transfer paper, more particularly an electrophotographic transfer paper to be used in an electrophotographic apparatus, electrostatic recording apparatus or the like, which prevents uneven discharge under low humidity conditions, and produces no faulty images and shows satisfactory paper transportability under high humidity conditions.
2. Related Background Art
Heretofore, color recording apparatus for electrophotography have been increasingly digitized to produce images of higher qualities, which needs a electrophotographic transfer paper to exhibit more uniform transferability. The characteristics of the transfer paper which affect toner transferability are electric resistance and surface properties of paper.
Resistance of the transfer paper is one of the most important properties in the art of electrophotography, and has been controlled at 1xc3x97109 to 1xc3x971011 xcexa9cm as surface resistivity and volume resistivity with the aid of an agent to improve electroconductivity. However, it is very difficult to control the conductivity, because it changes with water content of the paper, i.e., humidity in the atmosphere in which it is used. Today""s highly functional electrophotographic copiers and printers are designed so that the toner can be transferred to the transfer paper in an ambient environment under which they operate. As a result, they are designed to produce good images over a wide temperature/humidity condition range from a low-temperature/low-humidity condition of 10xc2x0 C./5% RH to a high-temperature/high-humidity condition of 30xc2x0 C./80% RH.
The transfer paper used to produce full-color images, having a basis weight of 60 to 100 g/m2, has a high moisture content, and is very sensitive to ambient humidity. As a result, it tends to have an excessive resistance, when handled at low-humidity circumstances, e.g., 15xc2x0 C./20% RH and 23xc2x0 C./5% RH, due to partial discharging phenomenon resulting from uneven charging on the paper surface, causing a problem known as a blank area in image. This type of problem results from discharging in an area where the transfer paper is separated, after an image is transferred thereto, from the photosensitive drum, and tends to be aggravated as resistance of the transfer paper increases.
Japanese Patent Application Laid-Open No. 8-171225 proposes adjustment of surface resistivity and volume resistivity of a transfer paper. However, the transfer paper having the above range of basis weight has a very high moisture content and is sensitive to ambient humidity. Keeping the resistance at a low level will cause another problem of leakage, when the paper is handled under a high humidity condition. In other words, the paper passes an excessive quantity of current, to prevent the image, or toner, from smoothly transferring thereto.
Japanese Patent Application Laid-Open No. 9-34158 proposes to increase smoothness of transfer paper surface, in order to secure uniform charging thereon.
However, when the transfer paper having the above range of basis weight is provided with a smoothness on the surface, an ordinary transfer paper may be damaged to loose firmness when coated and treated by, e.g., calendering. Usually, therefore, the paper of ordinary grade has not been treated for surface smoothness.
It is an object of the present invention to provide an electrophotographic transfer paper to resolve the above problems.
It is another object of the present invention to provide an electrophotographic transfer paper to be used in an electrophotographic apparatus, electrostatic recording apparatus or the like, which prevents uneven discharge under low humidity conditions, and produces no faulty images and shows sufficient paper transportability under high humidity conditions.
In accordance with the present invention, there is provided an electrophotographic transfer paper having a basis weight of 60 to 100 g/m2 comprising a coating layer provided at least on one side of the paper, and having an average surface centerline roughness (Ra) of 1.5 xcexcm or less, a volume resistivity of 1xc3x97109 xcexa9cm or more at 30xc2x0 C./80% RH and a paper stiffness of 16 cm or more.
The electrophotographic transfer paper of the present invention, having the above characteristics, prevents uneven discharge under low humidity conditions, and produces no faulty images and shows sufficient paper transportability under high humidity conditions.
The average centerline roughness (Ra) is determined in accordance with JIS-B0601 (1982). The volume resistivity is determined in accordance with provision JIS K-6911.
The paper stiffness is determined by finding length of the specimen strip of certain shape between the upper end point from which it is suspended and the lower end, when the specimen is rotated by 90xc2x0 on the axis of the line passing the upper end point in such a way that the lower end droops in the direction opposite to the specimen rotational direction. It is determined in the direction perpendicular to the paper-making direction, in accordance with provision JIS-P8143.